Enable retrieval of app usage event by guid

The app_usage_events_controller does not include the standard
'define_routes' call to the base model controller since there's no need
to provide all CRUD operations.  That's fine but the existing base
controller will return enumerations with url's for individual events
that were not handled by the app_usage_events_controller.

This simply the ability to get usage events by guid so the urls returned
from the enumeration work.

Signed-off-by: Ruth Byers <rbyers@pivotallabs.com>
Signed-off-by: Chris Adams <cadams@pivotallabs.com>